Contaminating particles are a major source of yield loss in the semiconductor industry. Accordingly, instruments have long been used to detect, locate and size particles on semiconductor wafers. Presently, the practical detection limit of particles on bare silicon wafers is about 0.1 to 0.2 micrometers. Because the amount of scattered light from a particle is proportional to d.sup.6, where d is the particle diameter, a reduction in particle size by a factor of 2 results in a 64-fold decrease in scattered light. Because a reasonable overall detection rate of about one to three wafers per minute is desired, and the number of photons scattered from a moving laser beam gets to be small around 0.1 micrometers, detection runs into a practical limitation around this size.
However, even though particles smaller than 0.1 micrometers cannot be reliably detected by currently available instrumentation operating at an inspection rate of one to three wafers per minute, does not mean that they are necessarily harmless. Such very small particles, although substantially smaller than the present critical line widths, are still considered very detrimental on very thin films. Here the important parameter is particle size with respect to film thickness, rather than line width. These particles may cause pin holes in a thin film and reduced dielectric strength, thereby causing premature breakdown.
In the semiconductor industry, condensation nucleus counters are used to detect very small particles suspended in the air of a clean room. This type of detector works on the principle that particles in free space act as nucleation centers for supersaturated vapor and can thus be artificially enlarged by condensing liquid onto the particles so as to form droplets that can be detected. An airstream is saturated with, for example, alcohol vapor, then enters a condensing region, where droplets form around any particles suspended in the airstream and grow in size until they reach a detection chamber. There the particles are detected and counted by virtue of the light scattered from the droplets. Typically, condensation nucleus counters can detect particles suspended in air down to a size on the order of 0.01 micrometers. It is, however, difficult to control the growth process so as to maintain adequate size discrimination between the original particles.
In U.S. Pat. No. 4,314,474, Dermarderosian describes a method in which an inert fluorocarbon vapor is condensed on a test surface in order to detect cracks, fissures and other such faults on the surface. Liquid fluorocarbon contained in a flask is heated to a gentle boil while an inert gas, such as air or nitrogen, is bubbled through the liquid. A mild flow of vapor is carried from the flask to the test surface by a vapor tube, the free end of which is held 1/2 inch to 1 inch (approx. 2 cm) away from the test surface. The fluorocarbon has a surface tension sufficiently low that as it condenses it wets the surface, forming a layer of uniform thickness. Detection is visual and may be made with the aid of a microscope and relies on the fact that faulted regions absorb comparatively more of the incident light than unfaulted regions. Defects on the order of one micrometer in size are visible.
In U.S. Pat. No. 3,580,066, Pliskin describes a method of determining the completeness of oxide etching of via holes in a silicon member surface, in which the silicon member is cooled while a stream of moist gas is directed onto its surface. The stream of moist gas is produced by bubbling dry nitrogen through deionized water. Condensation in the holes is in the form of a thin film over residual oxide but beads into droplets over bare silicon.
It is an object of the present invention to provide an apparatus for detecting particles and surface features smaller than 0.1 micrometers on a test surface.
It is another object of the invention to provide a method for estimating the sizes of detected particles, as well as characterize the types of particles present on the test surface.